AGEING
 An increase in the proportion of fibres coexpressing MHCI and -IIa and MHC-IIa and -IIb has been found in the vastus lateralis m. of old subjects (~70 yr-old). Similar findings for the biceps (Klitgaard et. al 1989).  In the very old (80+ years) ~ 50% of all fibers examined coexpresses two or three MHC isoforms (Andersen et al.1999).  Therefore ageing leads to an increased coexpression of different MHC isoforms. (ongoing fibre equilibrium?)  These differences, however, are region specific.

Summary of transformations
Increased use
 Type IIB (FG) fibres seem to be lost both a result of sprint as well as endurance training  MHC-IIB is probably the µdefault gene¶ which tranforms into MHC-IIA (FOG) following increased usage  Sprint training results also in a decrease in MHCIIB but also of MHC-I, thus two paths for MHCIIA expression may exist:

IpIIA nIIB

Transformations (cont.)
Decreased use
Spinal cord injury patients display a reduction of MHC-I isoforms and an increase in MHC-IIB. A reduction in proportion of type I fibres observed after cast immobilisation Confounding evidence after bed rest. Therefore SOME evidence for slow p fast transformation exists, though issue not resolved